Can I add my own deodorizer or a bag of ice to the tank to keep it fresh?

Do not add your own deodorizer or ice to the holding tank. The rental company has already added a measured amount of deodorizer that is designed to work for the rental period. Adding a different chemical can cause an adverse reaction, like foaming or gumming up the tank contents, which can damage the rental company's vacuum truck. The ice is not necessary for odor control and will just melt, adding extra water to the tank and reducing its effective capacity.

For your event, you have rented one standard portable toilet for 50 people over six hours. With alcohol involved, this is a heavy load for a single unit. The standard industry guideline for events is about one unit per 50 to 75 people for a four-hour event. Your event is longer and includes a keg, so the unit will be used more heavily. The warmth will also accelerate odor.

The best thing you can do is ask your guests to close the lid after use. This helps contain odors and keeps flies out. If you are concerned about the unit's condition, contact the rental company directly. They can confirm their policy and might offer an optional mid-rental service visit for an extra charge, which involves a full pumping and cleaning.